Metal carbides are metal compounds containing carbon in the interstices of the metal grid. In addition to possessing metallurgical properties, such as hardness and exceptional mechanical strength, these materials also have interesting catalytic properties. For example, molybdenum carbide and tungsten carbide possess catalytic properties that vary from corresponding metals and are similar to those found in metals such as platinum, palladium and rhodium, which are all more expensive.
Transition metal carbides are traditionally synthesized by carburization of metal oxides. Carburization can be achieved several different ways such as, for example, carbothermal reduction, electrochemical synthesis, arc melting with graphite, thermal decomposition of diethylenitriamine oxometal compounds, and reduction of oxides by H2 or mixtures of a hydrocarbon gas such as methane as the carburizing gas. Another method uses propane instead of methane mixed with H2 gas to obtain metal carbides.
Transition metal carbides were traditionally investigated for their mechanical hardness and high melting points for steel hardening, but they are also now used as catalysts for ammonia synthesis and decomposition, hydrogenolysis, isomerization, methanation, and hydroprocessing. Some molybdenum and tungsten-carbides behave similar to platinum in their catalytic properties. Catalytic activity of the carbides arises from the carbon atoms, which comprise up to 50% of their crystal structure, and which increase the metal-metal distance thus increasing the d-band electron density at the Fermi level of the transition metals (see FIG. 1). The ability of some carbides to mimic platinum has created interest in using them as a noble-metal-free replacement for the platinum/carbon electrocatalyst used in fuel cells.
Some methods to prepare carbides include a high temperature reaction. Transition metal carbides are traditionally synthesized by carburization of metal oxides. As discussed above, carburization can be achieved several different ways, for example, (a) carbothermal reduction, (b) electrochemical synthesis, (c) arc melting with graphite, (d) thermal decomposition of diethylenitriamine oxometal compounds and (e) reduction of oxides by H2 or mixtures of H2 with hydrocarbon gas as the carburizing gas. However, these traditional methods require multiple steps, hydrocarbons and the use of carbon monoxide. The inefficiency of multistep processes makes these approaches inefficient and expensive. Further, the use of flammable hydrocarbons in these traditional methods increases the risk of fire and explosion and the use of carbon monoxide requires implementation of expensive safety equipment and disposal protocols. The need for better approaches to prepare carbides is manifest.